Conservative behaviour

Liddicoat, M.I., D.R. Turner, and M. Whitfield. 1983. Conservative behaviour of boron in the Tamar estuary. Estuar. Coastal Shelf Sci. 17 467-472. 

The CCcu appeared to be linearly dependent on the concentration factor only in the upper part of the estuary. No increase in the CCcu couW be observed after concentration at higher salinities. The riverine colloidal material apparently coagulates to particles and floes at increasing salinity in the upper part of the estuary. These floes are retained by the 0.45 ym filter, thus no longer contributing to the complexation capacity of the "dissolved" fraction. This explains the non conservative behaviour of the CCcu n this part of the estuary (see later). Samples taken from the north Atlantic Ocean did not show an increase in CCcu> even aftar a concentration factor 200 times. 

The samples were collected from the F/S Valdivia cruise during 1981 (see Degens, 1982). In Figure 13 the distribution of DOC with increasing salinity in the Elbe, Weser and the Ems are depicted. The plots are similar to those reported previously (e.g. Mantoura and Woodward, 1983) and are indicative of conservative behaviour. 

Mantoura, R.F.C. and Woodward, E.M.S., 1983. Conservative behaviour of riverine dissolved organic carbon in the Severn estuary chemical and geochemical... 

Thallium (Tl), which appears to exhibit conservative behaviour in seawater, has two potential oxidation states. As Tl1, thallium is very weakly complexed in solution. In contrast, Tl111 should be strongly hydrolysed in solution ([T13+]/[T13+]t — 10 20 5) with Tl(OH)3 as the dominant species over a very wide range of pH. The calculation of Turner et at. (1981) indicated thatTl111 is the thermodynamically favoured oxidation state at pH 8.2. Lower pH and p()2 would be favourable to Tl1 formation. Within the water column, pH can be considerably less than 8.2 and /)( )2 lower than 0.20 atm. In view of these factors, and the observation that redox disequilibrium in seawater is not uncommon, the oxidation state of Tl in seawater is somewhat uncertain. The existence of Tl in solution as Tl+, a very weakly interactive ion, would reasonably explain the conservative behaviour of Tl in seawater. However, the extremely strong solution complexation of Tl3+ suggests that Tl3+ may be substantially less particle reactive than other Group 13 elements (with the exception of boron). 

Some definitions help in the interpretation of chemical phenomena in the ocean. Conservative behaviour signifies that the concentration of a constituent or absolute magnitude of a property varies only due to mixing processes. Components or parameters that behave in this manner can be used as conservative indices of mixing. Examples are salinity and potential temperature, the definitions for which are presented in subsequent sections. In contrast, non-conservative behaviour indicates that the concentration of a constituent may vary as a result of biological or chemical processes. Examples of parameters that behave non-conserv-atively are dissolved oxygen and pH. 

The major constituents in seawater are conventionally taken to be those elements present in typical oceanic water of salinity 35 that have a concentration greater than 1 mg kg excluding Si, which is an important nutrient in the marine environment. The concentrations and main species of these elements are presented in Table 1. One of the most significant observations from the Challenger expedition of 1872-1876 was that these major components existed in constant relative amounts. As already explained, this feature was exploited for salinity determinations. Inter-element ratios are generally constant, and often expressed as a ratio to Cl%o as shown in Table 1. This implies conservative behaviour, with concentrations depending solely upon mixing processes, and indeed, salinity itself is a conservative index. 

Not all the major constituents consistently exhibit conservative behaviour in the ocean. The most notable departures occur in deepwaters, where Ca and HCO3 exhibit anomalously high concentrations due to the dissolution of calcite. The concept of relative constant composition does not apply in a number of atypical environments associated with boundary regions. Inter-element ratios for major constituents can be quite different in estuaries and near hydrothermal vents. Obviously, these are not solutions of sea salt with the implication that accuracy of salinity measurements by chemical and conductometric means is limited. 

If the concentration of the measured component is, like salinity, controlled by simple physical mixing, the relationship will be linear (Fig. 6.3). This is called conservative behaviour and may occur with riverine concentrations higher than, or lower than, those in seawater (Fig. 6.3). By contrast, if there is addition of the component, unrelated to salinity change, the data will plot above the conservative mixing line (Fig. 6.3). Similarly, if there is removal of the component, the data will plot below the conservative mixing line (Fig. 6.3). In most cases, removal or input of a component will occur at low salinities and the data will approach the conservative line at higher salinity (Fig. 6.4). Extrapolation of such a quasiconservative line back to zero salinity can provide, by comparison with the measured zero salinity concentration, an estimate of the extent of removal (Fig. 6.4a) or release (Fig. 6.4b) of the component. 

Fig. 6.4 (a) Dissolved iron versus salinity in the Merrimack Estuary (eastern USA), illustrating non-conservative behaviour. Linear extrapolation (LE) of high-salinity iron data to zero salinity gives an estimate of 60% low-salinity removal of iron (after Boyle el d. 1974). (b) Dissolved barium versus salinity in the Chesapeake Bay (eastern USA). In this case linear extrapolation (LE) of high-salinity barium data to zero salinity indicates low-salinity release of barium (after Coffey el at. f 997), with permission from Elsevier Science. 

Biological cycling not only removes some ions from surface waters, it also transforms them. The stable form of iodine (I) in seawater is iodate (IOf), but biological cycling results in the formation of iodide (F) in surface waters, because the production rate of the reduced species is faster than the rate of its oxidation. Biological uptake of IOf in surface water results in a nutrient-like profile, contrasting with the conservative behaviour of most halide ions, for example CP and Br . The biological demand for NCp also involves transformation. Phytoplankton take up NO3 and reduce it to the -3 oxidation state (see Box 4.3 Fig. 5.12) for... 

Calculations by Kerdijk (1931) of dispersion processes indicated that chloride, showing conservative behaviour, will appear in the adjacent polders in approx, the year 2100, the heavy metals 1-3 centuries, and pesticides several thousand years later. 

The surface charge properties of river-borne humic compounds and their salinity and pH dependence have also been used to explain the non-conservative behaviour of humic compounds in estuaries (Hair and Basset, 1973 Eckert and Sholkovitz, 1976). Tschapek and Wasowski (1976) measured the surface activity of Na humates and concluded from the corresponding Gibbsian plots that (a) the surface area of the humic molecules at the water air interphase is 62—66 A and (b) that the surface active molecules were not polyvalent. 

Becdell, J.D.C. and Rehman, T. (1999) Explaining farmers conservation behaviour Why do fanners behave the way they do Journal of Environmental Management 57, 165-176. 

---